One class of flow meter and flow controller employs a body configured to provide for laminar fluid flow therethrough. Flowing fluid from a conduit system enters the inlet process connection of the meter flow body, passes through a laminar flow assembly, exits through an outlet process connection of the flow body, and continues its flow within the conduit system. In passing through the laminar flow assembly, the fluid creates a pressure drop, P1−P2, between an inlet pressure port and the outlet pressure port arranged on opposite sides of the laminar flow assembly that is sensed by a differential pressure sensor. Volumetric flow meters based upon the laminar flow principle are described in Spitzer, D. W., “Flow Measurement,” Instrument Society of America, Research Triangle Park, N.C., 1991, Chapter 8.
In the case wherein the laminar flow sensor is to be a mass flow sensor, the flowing fluid in the conduit system enters the inlet process connection of the flow body, and most of the total mass flow rate passes through the laminar flow assembly, exits through the outlet process connection, and flows back into the conduit. The flow through the laminar flow assembly is typically the majority fraction, M1, of the total mass flow rate, M.
In passing through the laminar flow assembly, the fluid creates a pressure drop, P2−P1, between the inlet pressure port (P1) and the outlet pressure port (P2) which forces a (typically) minority fraction, M2, of the total mass flow rate to enter the inlet pressure port, pass through a capillary tube in a mass flow sensor, and exit the outlet pressure port. As might be expected, in the case of the previously described volumetric flow meter, there is no such flow entering or exiting the inlet or outlet pressure ports, respectively.
In a mass flow meter where the flow of the fluid passing through the capillary tube and the laminar flow assembly are both nearly purely laminar and of substantially the same temperature, then the ratio, M1/M2, of the mass flow rate through the laminar flow assembly to the mass flow rate measured by the mass flow sensor is a constant, which is independent of flow rate and any fluid properties the following holds: M=M1+M2=M2 (1+M1/M2)=Constant×M2. Thus, the measurement of the mass flow rate, M2, through the sensor delivers a measurement of the total mass flow rate, M, through the flow body. Such capillary thermal mass flow sensors are more fully described in U.S. Pat. Nos. 4,487,062 and 4,800,754 and in Dieball, A., “Mass Flow Controllers Enter the Mainstream,” Sensors Magazine, August, 2000, pp. 14–21.
The mass flow sensor for said instruments may comprise upstream and downstream and upstream resistance temperature detector (RTD) type sensors. Other suitable types of thermal sensors include micro-thermal mass flow sensors based on thermal resistive, thermal electric, thermal electronic, pyroelectric or frequency analog transducing principles as described in Webster, John G., Mechanical Variables Measurement, CRC Press, Boca Raton, 2000, Chapter 9.9.
As alluded to above, a laminar flow sensor body can alternatively be used as a volumetric flow meter or can become a mass flow meter adding absolute pressure and absolute temperature sensors within the flow body to compute the fluid density p (e.g., in units of kilograms per cubic meter), which when multiplied by the volumetric flow rate, Q, yields the total mass flow rate, M (i.e., M=ρQ). Furthermore, the volumetric flow meter and mass flow meters may be configured as volumetric or mass flow controllers, respectively, when an integral flow control valve is supplied to the flow body.
However configured, laminar flow assemblies include one or more flow channels with dimensions sufficiently small that the passage of the fluid through them is laminar. Usually, the laminar flow assembly has either a transverse-flow geometry or an axial-flow geometry. One known laminar flow assembly with a transverse-flow geometry includes a plurality of annular disks fabricated of thin metal sheet stock compressively stacked together. Each such disk has one or more generally radially directed laminar flow channels chemically etched, or otherwise etched or fabricated, into one facet of the disk, about half way through the thickness of the disk. A second known transverse flow laminar flow assembly has its open disks fabricated of thin metal sheet stock stacked together, wherein the flow enters a relatively large entry channel on one side of every open disk; passes through a multiplicity of chemically etched, or otherwise etched or fabricated, small substantially rectangular laminar flow channels on one of the facets of each disk, all of which are rectilinearly directed (as opposed to radially directed) through the central portion of each open disk; and exits a relatively large exit channel on the opposite side of every open disk. In another version of the above second known transverse laminar flow assembly, disks between each said open disk act as gates which direct the flow in a serpentine-like pattern through the laminar flow assembly stack. If chemically etched, the laminar flow channels of the two above laminar flow assemblies have a substantially rectangular cross section, but with radii at their two bottom corners. Other known laminar flow assemblies with a transverse-flow geometry have alternative configurations designed to provide transverse flow paths through laminar flow channels of various shapes.
Known laminar flow assemblies with an axial flow geometry typically have a plug-like port blocking the entire flow body bore of the instrument in which one or more laminar flow channels direct their flow generally parallel to the axis of the flow body bore. Known laminar-flow-channel configurations of this type include one or more: porous plugs, usually consisting of sintered metallic particles; capillary tubes with small internal diameters; machined holes or grooves; or spaced plates stacked parallel to the flow body's axis.
When compared with those with laminar flow assemblies having an axial-flow geometry, those with a transverse-flow geometry offer advantages including: compactness; ease of fabricating assemblies accommodating different total flow rates; and less dependence on flow disturbances or non-uniformities upstream of the laminar flow assembly. Accordingly, there continues to be a particular interest in developing improved transverse-flow laminar flow assemblies for use in flow meters and controllers.